Single layer and few-layer (i.e. ten or fewer layers) graphenes exhibit a two dimensional carbon lattice structure with outstanding properties including high surface area as well as strong electronic, mechanical, thermal and chemical properties1,2. These properties have created considerable interest throughout the scientific community in recent years.
The first discovery of graphene was carried out by scotch taping peeling, although this approach can obtain pure graphene sheets, the process is not economical and impossible for mass production. The use of chemical vapour deposition (CVD) for production of few layer graphene has shown promise1,3,4. However, CVD produced graphene exhibits a low purity mixture of amorphous carbon. Many applications of graphene require large scale, high yield processes applicable to macroscale deployment. Currently, the most prominent technique for the scalable production of few-layer graphene is the chemical reduction or thermal treatment of graphene oxide (GO) from Hummer's method5-11. However, the oxidization process also exposes a large number of structural defects within the graphene sheets that compromise some of the properties and the unique morphology of the pristine two dimensional hexagonal carbon lattices11-14. Further, the multistep process, the concentrated acids used in oxidization and the high heat or harsh chemicals needed to reduce GO increase the economic, safety and environmental costs involved in large scale production15.
The drawbacks of the GO process have encouraged the pursuit of easily scalable processes to produce graphene with low basal plane and edge defects. For example, it has been shown that sonication of graphite with a solvent or surfactant can produce graphene flakes with low defect concentration. However, challenges associated with this method include low yield and purification difficulties. Exfoliation methods using organic solvents containing aromatic donors such as ortho-dichlorobenzene, n-methylpyrrolidone and benzylamine have shown stable dispersions up to 1 mg/mL through extended low power bath sonication, but are expensive and require special handling16-19. Surfactant based methods have also been investigated for large scale production, but are currently limited by low concentrations, of up to 0.05 mg/mL20,21. Longer sonication periods (400 hours) were shown to increase exfoliation concentration up to 0.3 mg/ml22 using sodium cholate. However, some surfactants exhibit bioaccumulation and are capable of adsorbing to proteins, disrupting enzyme function and causing organ damage23. The adverse cytotoxic health effects are coupled with potential environmental issues. Further, the large-scale application of surfactants creates significant accumulation in the water table, leading to the need for purification and treatment procedures to limit mammalian exposure23-25. These waste water treatments can add cost to the exfoliation process, reducing value.
More recently, Fan et al. have suggested the use of Gum Arabic for the exfoliation of graphene from graphite (J. Fan et al., J. Mater. Chem., 2012, 22, 13764-13772). The entire contents of this reference are incorporated herein by reference. However, the method disclosed in this reference results in a Gum Arabic/graphene complex, which is then treated to form a further complex with Ag. This reference does not teach a method for obtaining pure graphene.
Thus, the known graphene production methods involve various drawbacks. For example, the known methods often result in graphene of low purity, in that the resulting graphene contains a high amount of dispersant residues. The known methods are also often difficult to scale up and/or involve complex steps or equipment. In addition, the known methods are often not “environmentally friendly” in that they involve reactants or conditions that are hazardous or that may result in environmental damage.
There exists a need to alleviate at least one of the known drawbacks associated with known graphene production methods.